Episode 139 - Naruto Shippuuden (dub)

Tobi claims to be Madara Uchiha. He tells Sasuke he isn't an enemy. He reveals a shocking truth: he knows everything about Itachi. Sasuke lashes out, his Amaterasu activating automatically to kill Tobi. Tobi survives, proving his power is beyond comprehension. The Tragic Truth Tobi begins to unravel the lie Sasuke lived. Itachi wasn't a monster. He was a martyr. He killed the clan to prevent a civil war.

The battlefield is eerily quiet. Sasuke Uchiha stands over the lifeless body of his brother, Itachi. Rain washes away the blood and the years of hatred. Sasuke collapses from exhaustion, his mission finally complete—or so he thinks. A New Guardian Naruto Shippuuden (Dub) Episode 139

He spared Sasuke because he loved him more than the village. The Weight of the Truth Tobi claims to be Madara Uchiha